1. Field of the Invention
The present invention relates to a pattern inspection apparatus for a sample to which a pattern is formed, for example, a photomask for a semiconductor integrated circuit.
2. Description of the Related Art
In a manufacturing process of a large-scale integration (LSI), a reduction exposure apparatus for circuit pattern transferring (stepper) uses a photomask obtained by enlarging a circuit pattern fourfold- or fivefold-wide as an original plate. The demand for the integrity to the photomask (pattern accuracy or zero defects) is increasing year by year. In recent years, pattern transferring is performed in the vicinity of the limit resolution of the stepper by hyperfine/high integration, and the highly accurate photomask is essential for the device manufacturing.
Above all, improvement in performance of the mask inspection apparatus which detects defects in the hyperfine pattern is a must in improvement of short-term development/manufacturing yield of the advanced semiconductor device. FIG. 1 shows an example of such a pattern inspection apparatus.
This apparatus enlarges the pattern formed on the photomask 1 by using a microscope or the like, this enlarged pattern is divided into thin stripes having a width (W) of approximately 200 μm as shown in FIG. 2, and this stripe is continuously (actually, a table continuously moves) scanned, thereby inspecting defects.
Again referring to FIG. 1, the photomask 1 is mounted on an XYθ table 2, and a pattern formed on the photomask 1 is illuminated by an appropriate light source 3. The light transmitted through the photomask 1 enters a photodiode array 5 through magnification optics 4, and an optical image of the pattern is formed on the photodiode array 5.
The optical image of the pattern formed on the photodiode array 5 is photo-electrically converted by the photodiode array 5, and subjected to A/D conversion by a measured pattern data acquisition circuit (“measured data acquisition” in FIG. 1) 6. Measurement pattern data outputted from this measured pattern data acquisition circuit 6 is supplied to a reference data generation circuit (“reference data generator” in FIG. 1) 8 together with data indicative of a position of the photomask 1 on the XYθ table 2 outputted from a positional data acquisition circuit 7.
On the other hand, pattern design data used in pattern formation to the photomask 1 is read to a bit pattern generation circuit (“bit pattern generator” in FIG. 1) 11 from a magnetic disc 9 through a control computer 10. The bit pattern generation circuit 11 converts the pattern design data into bit pattern data in units of, e.g., pixels, and supplies this bit pattern data to the reference data generation circuit 8.
The reference data generation circuit 8 generates multiple-valued reference data by applying appropriate filtering processing to the supplied bit pattern data of the figure. Since the measured pattern data obtained from the measured pattern data acquisition circuit 6 is blurred due to the resolving characteristic of the magnification optics 4, the aperture effect of the photo diode array 5 or coherence between adjacent pixels, the multiple-valued data is obtained by applying the filtering processing to the data on the design side so that it can be readily compared with the measured pattern data.
A comparison circuit 12 compares the measured pattern data with the multiple-valued reference data in accordance with an appropriate algorithm, and determines existence of a defect when the data does not match.
Meanwhile, emergence of an LSI with a higher integration is demanded in recent years. Based on this, further improvement in the resolution of an optical aligner is demanded. As means for realizing these demands, there is proposed provision of a phase shifting pattern which utilizes coherence of the light in the photomask.
That is, as shown in FIG. 3, the pattern formed on the photomask 1 is divided into a peripheral pattern 21 and a circuit pattern 22. The circuit pattern 22 is divided into a logic controller part 23 and a memory part 24. The phase shifting pattern must be formed on a part of the circuit pattern 22 at which a fine pattern is required in particular.
In a usual photomask, a chrome layer having a light shielding function on a surface of a glass substrate is provided with a predetermined shape (which will be referred to as a chrome pattern hereinafter). The phase shifting pattern is usually formed of a translucent material such as SiO2.
Various types are considered for the structure of the phase shifting; an alternating phase shifting type shown in FIG. 4A, assist pattern type shown in FIG. 4B, edge enhanced type shown in FIG. 4C, chrome-less type shown in FIG. 4D, attenuated phase shifting type shown in FIG. 4E or the like. Incidentally, in FIGS. 4A to 4E, reference numeral 25 denotes a glass substrate; 26, a chrome pattern; and 27, a phase shifting pattern.
Based on these circumstances, a function capable of accurately detecting a pattern defect including a defect of the phase shift pattern is required for the pattern inspection apparatus. In the conventional inspection apparatus shown in FIG. 1, however, if both the chrome pattern and the phase shift pattern exist in the photomask, there is a problem that defects of both the patterns cannot be simultaneously detected.
As described above, in the conventional pattern inspection apparatus which detects a pattern defect by comparing the pattern design data used for forming a pattern and the measured data actually measured, the light intensity profile observed in the photo diode array 5 is as shown in FIGS. 5A to 5E if both the chrome pattern 26 such as shown in FIGS. 4A to 4E and the phase shifting pattern 27 exist in the photomask.
Incidentally, FIGS. 4A to 4E and FIGS. 5A to 5E correspond to each other. Further, the light intensity profile when the pattern shown in FIGS. 4A to 4E is formed by only the chrome pattern 26 is as shown in FIGS. 6A to 6E.
When both the chrome pattern 26 and the phase shifting pattern 27 exist, the light intensities corresponding to the respective patterns are apparently different. That is, as shown in FIGS. 6C and 6D, there is a characteristic of a so-called tri-tone state that the light intensity greatly varies at the contour part of the phase shifting pattern 27 or the light intensity of the phase shifting pattern 27 becomes an intermediate value of the glass portion 25 and the chrome pattern 26.
For inspecting such a chrome/phase shifting pattern mixed mask, the light intensity of the pattern shown in, e.g., FIG. 7A along the line A—A becomes as shown in FIG. 7B in particular. In this case, the actual inspected pattern is within an allowance but the chrome pattern and the phase shifting pattern do not overlap in accordance with the design data due to the pattern displacement when forming the chrome pattern 26 and when forming the phase shifting pattern 27 or irregularities in the manufacturing process.
If an allowable defect is rigorously detected as a defect, an operator must later make judgment upon whether repair is necessary, which deteriorates the overall efficiency. Therefore, preventing an allowable defect from being pointed out greatly contributes to improvement in performance of the inspection apparatus.
For example, part B in FIG. 7A may generate the displacement or have the line width which is not finished according to the design data as with part B′ in FIG. 7C. In such a case, if the defect detection sensitivity is increased, that part is erroneously detected as a false defect. Therefore, there is a problem that the defect detection sensitivity cannot be satisfactorily obtained.
Further, if a corner portion of design data such as shown in FIG. 8A is rounded as with an observed pattern in FIG. 8B, there is generated a difference between these patterns as shown in FIG. 8C, and this is detected as a defect. Therefore, there is also a problem that the detection sensitivity cannot be satisfactorily obtained.
Therefore, there is demanded emergence of a pattern inspection apparatus which does not determine an allowable displacement between the both patterns as a false defect and can efficiently simultaneously inspect the both patterns even in a photomask having both the chrome pattern and the phase shifting pattern.